IIRC Aisin Warner originally specified 3309 as an acceptable fluid to replace the proprietary "lifetime fill" AW-1 fluid. If you look in Toyota and Volvo repair manuals, they do specify 3309. However, I believe AW eventually revised that guidance and recommended sticking with AW-1 AND abandoning the "filled for life" concept.
Confusing matters further is that sometime around 2010 the v2 of the TF-80SC transmission came out, which specifies an entirely different fluid - 3324? I think that's it. The two fluids - 3309 and 3324 - are wholly incompatible.
I don't know why 3309 was the original go-to. AW1 and 3309 have very dissimilar viscosities, AW1 being much thinner. Maybe simply because it was available? I always suspected that between 2005 and 2009 they became aware of what most good mechanics had already determined... 3309 is problematic and that's why they switched to 3324, but I don't know.
IMO, even though AW originally specified 3309 in the TF-80SC you're foolish to use it. Pretty sure Saab drivers figured that out right quick. Stick with the factory AW1. Switching to 3309 increases wear throughout the transmission and causes long-term problems. Probably hurts fuel economy too.
I use Pentosin ATF1 LV in my Aero which is drop-in for AW1. 145k and it's doing great.